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Inelastic collisions in natural H2 are studied from the experimental and theoretical points of view between 10 and 140 K. Rotational populations and number densities measured by Raman spectroscopy along supersonic expansions of H2 provide the link between experimental and theoretical rotranslational state-to-state rate coefficients of H2 in the vibrational ground state. These rates are calculated in the close- scattering approach with the MOLSCAT code employing a recent ab initio H2–H2 potential. The calculated rates are assessed by means of a master equation describing the time evolution of the experimental rotational populations. The feasibility for obtaining the rates on the sole basis of the experiment is discussed. The dominant processes j1j2→j1′j2′ in the investigated thermal range are found to be 21→01>30→12>31→11, proving the importance of double processes such as 30→12. Good agreement is found between theory and experiment, as well as with earlier ultrasonic measurements of relaxation times. A spectral representation is proposed in order to visualize quantitatively the collisional contributions in any nonequilibrium time evolving process. [ABSTRACT FROM AUTHOR]

We report an experimental determination of the k00→02 rate coefficient for inelastic H2:H2 collisions in the temperature range from 2 to 110 K based on Raman spectroscopy data in supersonic expansions of para-H2. For this purpose a more accurate method for inverting the master equation of rotational populations is presented. The procedure permits us to reduce the measured k00→02 rate coefficient to the corresponding σ00→02 cross section in the range of precollisional energy from 360 to 600 cm-1. Numerical calculations of σ00→02 carried out in the frame of the coupled channel method are also reported for different intermolecular potentials of H2. A good agreement is found between the experimental cross section and the numerical one derived from Diep and Johnson’s potential [J. Chem. Phys. 112, 4465 (2000)]. [ABSTRACT FROM AUTHOR]

We present an experimental determination of state-to-state rotational-translational (RT) rate constants of N[SUB2]:He collisions in the vibrational ground state as a function of temperature in the range 3

Line coupling induced by collisions leads to drastic modifications of the spectral profile of the 0003–0000 of CO2 pressurized by helium. Calculation of these modifications have been performed by using two recent energy corrected sudden (ECS) formalisms. The two formulations lead to theoretical predictions rather similar and in good agreement with the available data over extended ranges of frequency and perturber pressure. It has been shown that a simultaneous fit of the pressure broadened linewidths and the near wing profile allows a more accurate determination of the basic ECS parameters. For that purpose, it has been necessary to extend the measurement of the broadened widths to high J values (up to J≊90). [ABSTRACT FROM AUTHOR]

Here we present the first line-by-line measurements of interference parameters Y0k describing line-mixing effects in the weak overlapping regime for He-broadened CO lines in the 0–1 and 0–2 bands at 296 K. A detailed analysis of the line shape at intermediate perturber pressures (up to about 10 atm) has been performed, starting from previous theoretical calculations, which has demonstrated the possibility of an individual measurement of Yk parameters. The method is based on the existence of a component of the line shift, quadratic with the perturber pressure (density) and proportional to Y0k. Comparison of our measurements with results predicted from coupled-states calculations shows good overall agreement. Linear pressure shifts have also been simultaneously measured. The uncertainty on the data is important since linear shifts are very small; however, some features can be considered as significant although we have no clear understanding of them. Possible explanations are discussed. [ABSTRACT FROM AUTHOR]

Investigates the accuracy of energy-corrected sudden (ECS) formalism for line shape calculations. Estimation of the coupled states of carbon dioxide-argon collisions; Observation on the ECS linewidths; Convergence of the coupled-state calculations.

Theory and experiment are combined in a novel approach aimed at establishing a set of two-body state-to-state rates for elementary processes ij→ℓm in low temperature N2:N2 collisions involving the rotational states i,j,ℓ,m. First, a set of 148 collision cross sections is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent potential energy surface for N2–N2. Then, the corresponding rates for the range of 2≤=T≤=50 K are derived from the cross sections. The link between theory and experiment, aimed at assessing the calculated rates, is a master equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (≈2×10-3 mm3) of N2 traveling along the axis of a supersonic jet. The calculated collisional rates are assessed experimentally in the range of 4≤=T≤=35 K by means of the master equation, and then are scaled by averaging over a large set of experimental data. The scaled rates account accurately for the evolution of the rotational populations measured in a wide range of conditions. Accuracy of 10% is estimated for the main scaled rates. [ABSTRACT FROM AUTHOR]